1. Field of the Invention
The present invention relates to an image pickup apparatus and to an image pickup system that employs the apparatus, and for example, relates to a color image pickup apparatus, which can select an operation for reading individual original pixel signals and an operation for adding the signals together and reading the result, and to an image pickup system that uses the apparatus.
2. Related Background Art
Nowadays, digital still cameras are available that are equipped with pickup devices that have two million pixels. The availability of pickup devices having such a high resolution is the result of the effort devoted to matching the image quality available with silver halide photographic equipment. These new cameras are especially used for static images. Conventional NTSC video cameras have only around four hundred thousand pixels, and reading speeds of about 13.5 MHz for interlaced scanning and 27 MHz for progressive scanning.
If a pickup device that has two million pixels is employed for a moving picture, the reading speed will be five times that attained by a four hundred thousand pixel device.
But when reading is performed at this speed, there is a considerable increase in the consumption of power and signals are degraded by the noise that is produced by the increased power consumption. Furthermore, device costs are also increased because more image processing memory is required.
To resolve this problem, a color image pickup apparatus was devised and is disclosed in Japanese Laid-open Patent Application No. 9-247689. In the embodiment that is described (FIG. 3, in the application), signals are read on 4×4 pixel block basis by thinning out signals for the same color are from each 4×4 pixel block, and are added together.
In this case, the problem is that the number of pixels in a 4×4 pixel block that are effectively used is reduced to ¼ of the total, and for the full pixel count, is reduced to 1/16 of the total. Therefore, for a device that has two million pixels, a resolution of only two million/16≈125 thousand pixels can be obtained. That is, the use efficiency is greatly deteriorated, and in actuality, such a device is used merely as a monitor.
Further, in the embodiment (FIG. 2 of the publication) in Japanese Laid-open Patent Application 9-247689 described above, a plurality of pixel signals are mixed and read. This process can not be implemented using a CCD (since a CCD employs charge-coupled transmissions, X-Y scan reading is difficult). In addition, the KTC noise along a vertical signal line is too large to perform the process using a semiconductor switch and a photodiode, so that a preferable S/N ratio can not be obtained.
As is described above, since in the prior art pixel signals are thinned out to be read in every 4×4 pixel block, a satisfactory resolution can not be acquired and a satisfactory S/N ratio can not be obtained.
Furthermore, to read a signal from a pixel, conventionally, a pixel signal for the first line is transferred to an upper memory, and a pixel signal for the second line is transferred to a lower memory. Therefore, the horizontal addition of signals is easy, while the vertical addition of signals or the oblique addition is difficult.